In general, in a method for obtaining a contact lens by polymerization in a forming mold (hereinafter, simply referred to as “molding method”) a polymerizing monomer is filled in between a female mold for forming the front side of the lens and a male mold for forming the back side of the lens, followed by heating or irradiation with ultraviolet ray or the like, thereby obtaining the finished contact lens. Since this method is simple and requires short time, it is widely employed as the method for manufacturing a disposable contact lens, which has become the mainstream of the contact lenses in recent years. When this manufacturing method is described literally, it may seem extremely simple. However, since the contact lens (hereinafter simply referred to as “lens”) is a specially controlled medical device, lens with high quality such as the shape and surface smoothness appropriate for wearing it on the eye, need be manufactured.
As a major problem to be solved in the molding method, polymerization shrinkage of the composition forming the lens can be mentioned for example. The lens material is formed by polymerizing the monomers and the like which are in the form of liquid, and thus the distance between the constituent molecules or the atoms become relatively close when the monomers and the like in the form of liquid changes into solid, thereby resulting in shrinkage in total volume. As the solution to such problem, a method involving the use of a mold provided with a flexible rim for allowing the lens to follow the polymerization shrinkage (Patent Literature 1), a method involving the use of an inert replaceable diluent so as to compensate the polymerization shrinkage by using a non-aqueous solvent which does not get involved with the polymerization (Patent Literature 2), a method involving the use of a mold possessing a feeder for holding an excess amount (compared with the space for forming the lens) of the material in order to compensate with the polymerization shrinkage (Patent Literature 3), can be mentioned.
Here, burrs and chips at the edge portion of the lens are also important problems to be solved. With respect to the front side and the back side of the lens, a smooth surface can be transferred to the product by using a mold with smooth surface. On the other hand, in the contacting portion of the male mold and the female mold, burrs and chips may occur if the lens forming portion is not certainly isolated from the excess material. As the solution to such problem, a structure having a knife edge arranged at the rim portion of either one of the molds so as to be inserted into the other mold (Patent Literature 4), a method involving a surface treatment to certainly adhere the excess monomer portion onto either one of the mold and the molded lens to adhere onto the other mold selectively in the case where the mold possessing a feeder is used (Patent Literatures 5, 6), and a method involving the use a forming mold having an interlocking means to substantially prevent side-to side-movement of the male mold in relation to the female mold (Patent Literature 7) can be mentioned.
Major problems seen in the molding method were solved by these techniques, and thus it became possible to supply lens at low cost. Accordingly, there is presently a demand for improving the value of the lens by providing new function to the lens by utilizing the features of the molding method, and applying a new lens material.
As an example of improvement in the value of the lens, a colored lens capable of objectively varying the color or the texture of the pupil of the eyes can be mentioned. Such lens is referred to as iris lens. As an example of the manufacturing method of such lens, a method involving the application of a colored monomer onto the portion of the surface of the lens-forming mold where the iris is to be formed, formation of the film, charging of the monomer component for lens into the mold, followed by polymerization and separation of the lens from the mold, thereby obtaining the colored lens (Patent Literature 8), and a method involving the manufacture of a lens having the colored portion embedded in the lens material (Patent Literature 9) can be mentioned. According to such methods, the colored portion would not protrude with respect to the surface of the lens, and thus the wearer of the lens would not have uncomfortable feelings directly on the eyelids and the like. However, since the colored portion may appear on the surface of the lens, the colored portion may peel off depending on the bonding strength with the material of the lens main body. This would cause irritation to the wearer. In addition, the wearer may feel the difference in the hardness between the colored portion and its peripheral portion, or stain may adhere to the colored portions.
Accordingly, colored lens having its colored portion coated with a coating layer has been suggested (Patent Literature 10). Regarding such lens, the colored portion is covered completely, and thus the problems mentioned above can be avoided. However, there is a problem regarding its manufacture, since the coating layer is formed in a separate process by graft polymerization via plasma surface treatment or UV irradiation. Introduction of another process of different type is expected to raise the manufacturing cost, and would be problematic since the molding method is characterized in the low manufacturing cost for the lens.
Here, most of the disposable lenses are soft lenses. The soft lens is suitable for molding method, since the soft lens has less lens species (especially the base curve) than those of the hard lens, and thus number of types of the mold required is less. This would allow cut in cost and mass production. Conventionally, material of the soft lens has obtained its oxygen permeability by its high water content. Here, the material has shifted to new lens material having higher oxygen permeability, which uses siloxane-containing macromonomer and the like. When the siloxane-containing macromonomer and the like is used, extremely high oxygen permeability can be obtained, however, another process such as a surface hydrophilic treatment (Patent Literature 11) is required.